A vehicle such as a work machine, wheel loader, backhoe loader, on- or off-highway truck, or the like is normally equipped with a number of wheels. Generally, the wheels are mounted in pairs on an axle. A friction-style service brake of a known type is associated either with each wheel or each axle. A “wet” axle, having axle oil or another fluid circulating within an enclosing structure, is an example of a common configuration in the industry.
Brake overheating is a very common and serious condition. The brakes can easily overheat when they are applied for a long duration, or a closely-spaced series of shorter duration applications. When a brake overheats, the two main resultant failure modes are axle oil failures (in a wet axle) and brake failures. Axle oil failure occurs because heat makes the axle oil less viscous and more “runny”, with the consequence that the oil does not flow and adhere as needed to properly lubricate axle components. The axle components then can wear prematurely. Brake failures fall, for the most part, into three categories related to the friction discs: glazing, warping, and carbonization.
Glazing is the term used when the friction disc surfaces become heated, perhaps at least partially melted, and lose relative friction with their partner discs. While glazing effects the characteristics of the brake initially, as the glazed brake is applied, the glazing becomes worn away to expose the friction material beneath and the brake can eventually regain close to normal function. Warping occurs when heat causes the friction discs to twist or melt. A warped disc only frictionally engages its partner disc at high or low points along the surface of the warped or partner disc, thus exacerbating the overheating problem at those points and possibly causing a loss of braking force. Finally, carbonization is when the material which makes up the surface of the friction discs loses structural integrity because of the heat and chunks or flakes off, causing similar ill effects as does warping.
Unlike glazing, both warping and carbonization damage the brake permanently, necessitating replacement of the damaged friction discs. Brakes are extremely difficult and time-consuming to access due to their location on the vehicle, and brake components are very costly. It is thus desirable to detect or predict an overheat condition before permanent damage occurs to the brakes.
Generally, brake overheating occurs when the vehicle is traveling down an incline and the operator is “riding” the brake to control the speed of the machine by resisting gravitational force. Brakes are normally sized to bring the vehicle to a stop on level ground with an application duration of 3–5 seconds. Tests in the field have shown that permanent brake damage from overheating can begin in as short as a 15–20 second application. Though on level ground the vehicle would have long since stopped by the 15-second mark, such a damage-threshold brake application duration is often exceeded by downhill-travelling vehicles and the brakes of such a machine can overheat. A warning or prediction system is needed to alert an operator of a present or impending brake overheat event so that permanent damage can be averted or minimized.
Prior art systems often use an axle oil temperature sensor to measure an actual temperature of the axle oil and compare that temperature to a range of allowable temperatures, alerting the operator if the temperature exceeds the range. However, the sensor may be located inconveniently far from the brake because of spacing and wiring constraints and consequently may not detect all instances of brake overheating due both to the distance of the sensor from the brake and to the unpredictable circulation of axle oil. Sensors can easily malfunction in the stressful environment inside the axle or brake, as well.
Also, even when the operator is alerted of overheat conditions, damage may have already begun to occur and may become irreversible in the time that it takes the operator to stop the vehicle to let the brakes cool. Therefore, a proactive warning is desirable as long as the number and frequency of warnings without an associated overheat event are small enough that the operator does not disregard the warning.
Additionally, cooling of an overheated brake sufficient to allow further use generally takes on the order of 15–30 seconds, depending upon the temperature of the brake and the setup of the vehicle. Operators are often held to a tight work cycle schedule and may wish to minimize stopped time. If the brakes are not permitted to fully cool before vehicle travel resumes, a low-level overheat situation results. While no permanent warping or carbonization may occur, such a constant low-level overheat may bring about an axle-oil failure as described above and allow catastrophic wear to axle components. Accordingly, a system which indicates to the operator when the brakes have cooled to an acceptable level would be of value to an operator trying to keep to a strict productivity schedule while still protecting the components of the vehicle.
Due to the difficulty and expense of brake repairs, it is also desirable for a log or history of overheat (or near-overheat) events to be kept. This allows the owner of the vehicle to identify a particular operator or work site with a high incidence of overheat events. The owner may then provide additional operator training or a vehicle modification, such as an axle oil cooler, to avoid further overheat events.
U.S. Pat. No. 5,079,947, issued Jan. 14, 1992 to Joachim Feldmann et al. (hereafter referenced as '947) discloses a method for the approximate determination of an average temperature of a device component of a brake device. The average temperature is determined by the deformation of the device component at the start of a brake actuation and at a point in time t1 following the start, as well as by a temperature-dependent change of the elasticity module of the material of the device component. '947 calculates the brake temperature indirectly, thus avoiding the temperature sensor difficulties described above. However, the '947 method requires that a number of specialized device sensors be placed near or on the brake components, thus negating the time, resource, and efficiency savings provided by the lack of a temperature sensor.